1. Field of the Invention
The present invention relates to a logic circuit which is for use in a digital-to-analog converter (hereinafter referred to simply as a D/A converter) or another circuit device and which is designed to output seven kinds of state signals on the basis of digital signals of three bits.
2. Description of the Related Art
D/A converters are a circuit which converts a digital signal into an analog signal, and are normally employed as an interface inserted between a digital circuit and an analog circuit. Among various types of D/A converters, there is a type which has a resistance ladder circuit structure and is designed specially for a high-speed D/A converter. When digital signals of n bits are converted by this type of D/A converter, the digital signals of lower m bits are used for controlling the current sources of the resistance ladder circuits of the lower m bits, and the digital signals of upper (n-m) bits are used for controlling the current sources of the current summing section of upper (n-m) bits. The upper current summing section serves to generate state signals of {2(n-m)-1} kinds from digital signals of upper (n-m) bits for controlling the current sources of {2(n-m)-1} kinds. That is, analog methods have been used for generating state signals, for example, state signals of {2(n-m)-1} kinds from digital data of (n-m) bits. Conventional state signal generating circuits have the problems that the element characteristics as well as the temperature characteristics of the resistors and current sources used in them can vary with the result that the switching timing of state may not be uniform.